\(= \sqrt {12 - \sqrt {{{(6 + 4\sqrt 2 )}^2}} } \)= \(\sqrt {12 - 6 - 4\sqrt 2 } = \sqrt {6 - 4\sqrt 2 } \)
\(= \sqrt {{2^2} + {{(\sqrt 2 )}^2} - 2\,.\,2\sqrt 2 } = 2 - \sqrt 2 \).